Page 188 - Handbook of Electrical Engineering

SWITCHGEAR AND MOTOR CONTROL CENTRES 171

Hence the peak making capacity of the 32 A MCCB is well in excess of the let-through peak
current of the 125 A MCCB.
l) Find the highest I-squared-t value for the upstream MCCB.
Locate two points P and Q on the curve of the upstream MCCB as follows,

2
Current Current Time in I t
Point in p.u. in amps Seconds
P 14 406 6 989016.0
Q 602 17,450 0.0016 487204.0

2
Hence I tat P exceeds thatatQ.
m) Calculate a suitable size for the load cable to satisfy the I-squared-t duty.
For XLPE cables the ‘k factor’ for the I-squared-t is 143. The cross-sectional area A is:-

2
(I t) 0.5 (9,89,016) 0.5 2
A = = = 7.42 mm
K 143
2
The next standard cross-sectional area is 10 mm .
n) Calculate the volt-drop in the load cable.
The usual limit to volt-drop in three-phase cables feeding static loads is 2.5% at full load.

1.732 × I flc × L(R cos φ + X sin φ)
Volt-drop =
1000
Where, I flc = 29 A, L = 15 m and φ = 54.5495 degrees.
2
For a 6 mm cable the volt-drop is found to be:-
1.732 × 29.0 × 15.0(3.91 × cos 54.5495 + 0.13 × sin 54.5495)
Volt-drop =
1000
= 2.504 + 0.0516 = 2.6 volts or 0.58% of 440 V
which is well within the limit of 2.5%.
o) Select the largest conductor size from the above calculations.
2
Comparing the conductor sizes found in m) and n) gives the larger as 10 mm , and this size
should be used.
p) Revise the calculation of the fault current I fd
The impedance Z c2 of the load cable is:-

15.0(2.31 + j 0.128)
Z c2 = = 0.0347 + j 0.00192 ohms
1000.0
Add Z c2 to Z fc to give the fault impedance Z fd as:-
Z fd = Z fc + Z c2 = 0.0143 + j 0.002759 + 0.0347 + j 0.00192

= 0.049 + j 0.00468 ohms